


Stuffed

by Nyarm



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, Big Brother Papyrus, Cum Inflation, Ectobiology, Fellcest - Freeform, M/M, Penetrative Sex, Sibling Incest, Underfell, kinkmeme fill
Language: English
Status: Completed
Published: 2016-08-03
Updated: 2016-08-03
Packaged: 2018-07-29 00:09:03
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,087
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7662532
Author URL: https://archiveofourown.org/users/Nyarm/pseuds/Nyarm
Summary: <blockquote class="userstuff">
              <p>prompt:</p><p>The reason Fell!Sans is so plump is that older brother Papyrus makes sure to regularly stuff him full. Monster food is concentrated magic, monster come is concentrated magic - ever since they were on the streets, Papyrus has made sure his brother is 'well-fed'. Even now that they have a house in Snowdin and regular income, he fills him up every few days - at both ends. (Sans should be a very willing participant, please.)</p>
            </blockquote>





	Stuffed

**Author's Note:**

> [Prompt](https://undertail.dreamwidth.org/256.html?thread=65536#cmt65536)

Sucking up the dredges of a mustard bottle did little to curb the curl of excitement that lay low in Sans’ non-existent gut. It was a Thursday, and while that may not have meant much to the other unlucky fucks wasting their time at Grillby’s, it meant a hell of a lot to Sans. Thursday was Sans’ favorite day. It was the day Papyrus had to deliver a weekly report to the capital and came home pissy enough to actually want to relax. Of course, that meant a great time for Sans as there was really only one way his older brother liked to relax.

They’d been doing this since they were scavengers roughing it out on the streets. When you couldn’t find food, being pumped full of cum was the next best thing. After all, both monster food and monster cum was made of concentrated magic and his big bro, his boss, would never let him starve. The process was so enjoyable that Papyrus tended to stuff him in excess, causing to him to become a bit chubby. Though his brother never minded, in fact, he seemed to enjoy how soft Sans was, especially because he knew how that softness came to be. Sans slurped up the very last drops of mustard, relishing in the flavor and consistency. It reminded him of Papyrus’ cum, thick and spicy, warming him up even when he was dripping with it from both ends. Sans got up and slapped the bottle on the bar before walking out without paying, ignoring the purple fire man’s squawking. He’d deal with that later.

The short walk had Sans constantly buffeted by brisk winds, but the chill just made him more eager for what awaited him at home. At least the walk was short and a scant few minutes had passed before he was unlocking his door and scurrying inside. His older brother was always in his room on this day, he rarely initiated sex for fear of pressuring Sans into anything with his domineering nature. Sans thought Papyrus sweet for it, but he could honestly not think of a single time he hadn’t been down for a fuck. He craved his brother’s touch, that much was undeniable. Even now, when they were living comfortably and there was no need to use sex as a tool against hunger, they kept on with this little ritual. His brother loved it and Sans adored it, so there was never any reason to stop.

Sans took the stairs two at a time, full of anticipation. God, he’d waited all week for this. Typically, they would fuck a few times a week, but Thursdays were the days where he was fucked with the sole intention of stuffing him full of cum. Sans scurried up to his brother’s room and upon noticing that the door was left suggestively ajar, tentatively pushed his way in. Papyrus was sitting on his bed, tending to his claws and clearly waiting for Sans.

“Boss,” Sans said, addressing his brother just the way he liked, all slow and reverent. Papyrus liked to be in charge when it came to their sex life, insisting he be called Boss in the bedroom, which was just fine by Sans. He loved it when his brother was dominant and controlling, it was hot as hell. His brother changed from Boss to Papyrus as soon as he entered their home, but he assumed his Royal Guard facade once again during their activities. Papyrus beckoned him over, so Sans came and kneeled before his brother, eyes level with his groin. He reached up to unfasten Boss’ pants, well acquainted with their routine. Boss’ cock was so enticing, it was a deep red and girthy and Sans never knew if he wanted it in his mouth or inside him more.

“So eager,” his big brother commented as Sans took him into his mouth and moaned, sucking furiously. 

“Mhm,” Sans nodded eagerly and tried to voice his agreement through another moan, muffled by dick. There was nothing he loved more than his brother’s cock, a hot and heavy weight in his mouth and so better tasting than mustard. Sans could feel his magic swirling in his pelvis, warm and ready to coalesce into an entrance, ready to be filled. 

“You’re always so hungry for this, Sans. Do you really love my cum that much?,” Boss wondered aloud, causing Sans to nod frantically. He could feel Boss cup the back of his head before thrusting hard into his mouth with a grunt, so Sans went limp. He loved having his brother’s cock pounding into any of his orifices and let his magic cushion its passage. He loved letting his brother have his way with him. He wanted to tell him to fuck his face harder, but, sadly, he could not with his voice muffled as it was. He resorted to just moaning his approval around the dick in his mouth, the vibrations inadvertently causing Boss to go harder for want of the pleasant sensation.

“Sans, ahh, are you ready for me to… ?” He asked, trailing off as his hips stuttered. Sans looked up at him and nodded eagerly. His brother was quick to finish, something that had embarrassed him at first, but Sans was so receptive that the insecurity faded fairly quickly. Sans didn’t mind it in the least, his brother’s crazy stamina just meant that Sans got far more cum far more times than he would with any other monster. His brother fucked into his mouth a few more times before he stopped and groaned, grinding into Sans’ mouth. A rush of thick red release flooded Sans’ mouth and he couldn’t help but suck as hard as he could, wanting to get every last drop of his Boss’ delicious cum.

“Hah! H-hold it in your mouth. Don’t swallow until I say so,” his brother said, commanding regardless of his breathless tone of voice. Sans whined, wanting nothing more more than to feel his brother’s cum settle low in his belly. He refrained however, and dutifully kept it in his mouth. At least he could taste it for longer, he reasoned, swishing it around with his tongue. He could taste the salty spiciness and had to admit that there was really no better flavor in the world than Boss’ cum. Mustard came close in tastiness, but it couldn’t beat the feeling of warm viscous fluid in Sans’ mouth, slippery, but thick. It was amazing. Sans moaned around the mouthful and Boss looked pleased. He tugged Sans up onto the bed and started undressing him. Sans took this as a sign to form his ecto-body as he aided his brother in stripping. When Sans’ formed flesh was bare, his brother started to caress his chubby belly with a pleased hum, touches leaving trails of sensation that made the heat between Sans’ legs flare up. Sans’ tummy had always seemed to be one of Boss’ favorite parts of Sans. If absorbed magic didn’t meld with that of its recipient and stayed its original color, Sans was sure the majority of his mass would be the deep blood red of his brother’s magic, rather than his own lighter hue.

When Boss’ sharp phalanges slipped down to caress the folds of his entrance, Sans almost gasped before remembering the load he carried in his mouth. He spread his legs in frustration instead, showing his eagerness to have his brother’s cock inside him. He made muffled sounds through his closed mouth to show his appreciation as those same deft phalanges slipped inside him before withdrawing, starting a slow rhythm to finger him to. Feeling his brother’s fingers rub against his wall was so good, twinges of pleasure shooting through Sans everytime they slipped a bit deeper than before. The liquid in Sans’ mouth prevented Sans from begging for Boss to just fuck him already, so Sans just tugged him closer while giving muffled whine.

“If you want something, you can just ask for it,” Boss teased, even though he was lining himself up with Sans’ entrance as they spoke. Sans couldn’t help but try to grind back onto him, trying to get him inside, but his brother’s hands cradled his ass and made sure he couldn’t.

“Mm- Nnh!” His grunt of irritation turned into a whimper as his brother pushed inside, the liquid release in his mouth getting more and more difficult to contain. The feeling of Boss’ thick hard cock pushing into him, spreading his walls as it went deeper and deeper almost made him open his mouth to moan. Boss always knew just what he wanted, just how to fuck him, thrusting hard, but slow at first, rubbing against every inch of Sans’ passage. Any sensation Sans may have garnered from his brother’s deft phalanges paled in comparison to what he got from being fucked. Every thrust left him wetter and hotter and wanting so much more. Waves of sensation emanated out from his pelvis every time his brother hilted. Boss’ hands groped and rubbed at his body, enjoying the give of his soft, bountiful formed flesh. When his brother abruptly changed the pace to shallow and fast, Sans didn’t know what to focus on. He felt like he was melting under hands so well acquainted with his body, so knowledgeable on how to pleasure him. The slick slide as he was penetrated again and again and the sound of his brother moaning above him while his clawed hands roamed Sans’ body… Sans didn’t think he could hold out for much longer. 

“Buh!” Sans shrieked a little in sudden pleasured surprise when his brother gripped his pelvis for leverage as he suddenly started to slam into him, some of the cum sloshing out of his mouth, some slipping down his throat. Sans choked a little, unable to find a moment of reprieve between Boss’ harsh thrusts and his newfound struggle to not lose anymore of the liquid.

“Sans, you’ve, mm,” He paused, moaning as Sans clenched up against the onslaught of thrusts, “You’ve made a mess,” his brother stated, his tone belying the fact that he wasn’t at all bothered, before dragging his tongue up the side of Sans’ face, lapping up all of the spilt liquid. He licked his way into Sans’ mouth, kissing him, sharing in the ejaculate kept warm by his little brother’s mouth. 

“You can swallow now. After all, I’m about to give you some more,” Boss said, fucking Sans still as he pulled away, “Besides, I want to hear just how much you love this.” Sans obeyed gratefully, and swallowed in a couple big gulps, moaning as his the cum settled low and heavy in his gut.

“Ahh, Boss, please cum inside me!” Sans wanted more, he wanted so, so much more. Being full of his brother’s essence was his favorite thing in the world and he couldn’t wait until he was filled up with it! Feeling the weight in his belly and the lingering taste had him desperate. Sans rocked back to meet the thrusts and he could feel Boss getting more and more excited as he did. He hoped he was close, Sans felt like he was about to burst. He could feel his older brother’s thrusts get harder and more erratic than before.

“Ahn! Boss!” Sans yelled, it was all too much and Sans felt himself clamp down on his brother’s cock as he came, moaning and grinding. He was desperate to ride out the sensation, milking his orgasm for all it was was worth as a rush of his fluid gushed out where he and his brother joined.

“Nnh, Sans!” Boss cried out, grinding down in turn and gripping Sans hips hard. His orgasm washed over him and Sans met his final rough thrusts even through the shivering of his overstimulated insides, eager for the warm red fluid that coated his insides in large spurts. He could feel it settle with the rest of the release in his gut, his magic drawing it further into his his body and mixing it into his own.

Papyrus pulled out and tugged Sans in for another kiss, giving him a brief reprieve. Sans put his forehead to his brother’s sternum, tired from their session. He rested a hand on Sans’s formed belly, “You aren’t tired yet, are you?” he asked darkly, trailing his hand down to Sans’ entrance.

“You’re still so empty. What kind of big brother would I be to let you go to bed without a full tummy?”

**Author's Note:**

> [i have literally never eaten mustard](http://pixie-pier.tumblr.com/)


End file.
